The present invention relates to a method for the reformation of an intermediate circuit capacitor of a converter formed from at least one electrolytic capacitor and an associated converter for converting an AC grid voltage to a direct voltage or vice versa.
Converters for converting alternating current to direct current and vice versa are used in many applications, for example, for coupling electrical grids with variable-speed drives, for energy exchange between two electrical grids, for photovoltaic and wind power plants and the like. To do this, power converters in different circuit topologies and configurations for different power and voltage ranges are known. In the medium-voltage and high-voltage range, multipoint converters are increasingly used, which have a circuit arrangement with controllable switching elements, preferably power semiconductor switches, which can be clocked at high frequencies to generate a plurality of voltage levels and handle higher voltages than the blocking capability of a single power semiconductor switch permits.
Some converters on the DC voltage side comprise a so-called DC voltage intermediate circuit with an intermediate circuit capacitor arranged therein. In frequency converters to supply power to drive motors or wind power generators, such an intermediate circuit capacitor is arranged between the grid-side rectifier and the load-side inverter and connected in parallel.
In many applications, the capacity of the intermediate circuit capacitor is substantial. In the case of some very high intermediate circuit voltages in the range of several hundred volts, series connections and/or parallel connections of capacitors, preferably electrolytic capacitors, are generally used to provide the required dielectric strength and to be able to store large charge quantities. Such series and/or parallel connections of electrolytic capacitors are referred to herein as the “intermediate circuit capacitor” in the same manner as a single capacitor.
Electrolytic capacitors are polarized capacitors whose anode electrode is made of a metal, in the case of aluminum electrolytic capacitors, for example, aluminum, to which an oxide layer is applied electrochemically as insulating layer, which forms the dielectric medium of the capacitor. The electrolyte is the cathode of the electrolytic capacitor. The quality of the oxide, that changes repeatedly in the course of the electrolytic capacitor production and subsequent use, determines the insulating properties of the dielectric medium. With prolonged storage of electrolytic capacitors in voltage-free condition, especially at elevated temperatures, or due to aging, the oxide degenerates, so that its DC conductivity increases. During recommissioning after non-use, the residual current, which flows as a leakage current through the capacitor, may be relatively high shortly after applying a direct voltage. In applications for converters, a very high residual current can lead to the destruction of the electrolytic capacitors of the intermediate circuit capacitor. To prevent this and minimize the interfering conductivity of the dielectric medium, it is necessary to repeat the forming process under voltage in order to repair the electrolytic capacitors after prolonged non-use and build up the oxide layer again. The repetition of the forming process is referred to herein as reformation or reforming.
To reform the electrolytic capacitors in converters, it is known to slowly ramp up the intermediate circuit of the converter with, for example, an adjustable DC voltage source to a desired reformation voltage. To do so, a technician must connect the voltage source via a special reformation device directly to the intermediate circuit and, for example, slowly adjust it upward according to the specifications of the manufacturers of the electrolytic capacitors. To this end, the converter module must be disconnected from all possible energy sources, in particular from a supply grid, which may optionally require measures from a control center. Furthermore, the technician must ensure by measuring that the converter is in fact switched off, i.e. is currentless and potential-free at the accessible connections. Furthermore, the technician must ensure that the DC voltage intermediate circuit capacitor is discharged before connecting the reformation device to the DC voltage intermediate circuit. For this purpose, physical or electrical access to the DC voltage intermediate circuit conductors is required. The reformation device must also be connected to an auxiliary voltage source of the converter system or must have its own DC voltage supply.
The reformation process can only be started when all the above measures have been taken. The reformation requires the use of highly qualified, trained personnel, and is time-consuming, costly and prone to human error. Due to the possibility of human errors, it presents a risk to human health of the maintenance staff and the proper functioning of the system.
Converters with an intermediate circuit capacitor of high capacitance cannot be connected to the grid without taking special measures, since the switch-on current could be excessively high. Such a switch-on current surge could damage or destroy the converter and the intermediate circuit capacitor. In case of adverse circuit impedances, too high a voltage can arise in the intermediate circuit capacitor, and adverse repercussions of the switch-on current surge on the supplying grid are possible.